Carrier signaling system



Aug. 18, 1931. H A, AFFEL 1,819,054

CARRIER SIGNALING SYSTEM Filed May 2 4, 1927 ave Vallone P ae Laiszlvzlradared @maz/fil@ y Sendai/zg of Subset Metwnrk ATTORNEY PatentedAug. 18, 1931 vunirse stares ear-snr ferries f HERMAN A. Arret, orRinenwoon, new JERSEY, Assienoa To' AMERICAN TELE,-`

PHONE AND TELEGRAPH COMPANY, A OOBPOBATIGN OF NEW YORK CARRIER SIGNALINGSYSTEM Application `filed 'May 24,

This invention relates to signal transmission systems, and particularlyto a method of operating such systems to minimize the effect of noiseupon the signal wave being ,5 transmitted, the said method beingcharacterized by an intentional predistortion of the signal wave, andtherestoration to its normal form after its transmission over or througha medium. The inventionkmay be applied to voice frequency transmissionsystems andto carrieror radio systems.

In the transmission of signals over a signal transmission system, suchas carrier or radio system, it is desirable, in order to avoiddistortion, that the system shall have'the same over-all equivalent atall frequencies within the range to be transmitted. In the systemsheretofore employed, van effort has been made to provide in each part ofthe sys- 201cm, including'the parts subject to external sources ofnoise, substantiallyV distortionless transmission of the frequencies inthe signalingwave. lt is well known that under normal conditionssome ofthe frequencies inthe original signal wave contain larger amounts ofenergy than others, and that the energyfrequency.distribution may be.further dis*- torted duringl the conversion in the transmitter from iairwavesinto electrical waves. In the, ordinary transmission system thenonuniform distribution thus obtained in the transmitteris preserved toa greater or'less extent throughout the system. i

This vinvention resides in a method of operating a. signal transmissionsystem, which consists in predistorting the signa-l wave at thetransmitting end-in such a manner as to ensureA a practically/constantamplitude for all frequencies inthe signaling wave and in' 40 restoring'the transmitted wave to its original format the receiving station by adistortion that is the complement of the distortion at the transmittingstation.

' ln `a transmission circuit involving a sende ing terminal, a line ortransmission circuit, and a receiving terminal, there are often one ermore points at which gain is introduced inthe circuit, usually by meansof vacuum tube amplifiers, which, of course, have a deli- 5o niteload-carrying capacity.- Also, those cir- 1927. Serial No.`193,902.

cuits areinvariably affected Vby noise from external sources. Such acircuit is usually arra. ed so that the ampliiication introduced in thecircuit will maintain the transmission level suiliciently high 'tooverride such noise. l have found that in such a circuit the energylevels may be better distributed and the eiective carrying capacityr ofthe amplifier more efficiently employed,fwith less over-allinterfere-nce, if there is provided at the sending end of th-e circuit acertain transmission-frequency distortion, and similarlyv atthereeiving` terminal a complementary `frequency distort-ion, both`properly related to the volnine-frequency characteristic of thetransmitting source and `the noise-,frequency characteristicof thedisturbance. The invention will be clearly understood from the followingdescription when readin connection with the attached drawings, of whichFigure l showsjschematically a voicefrequency circuit embodying theinvention; .Figs la, 17), lc and 1d are graphs illustrating themethodfin which the `invention is embodied; Fig. 2 represents schematically aradio circuit embodying*- the invention, and Fics. 2a, 2b, 2c and 2daregraphs illustrating the application ofthe invention in' Fig. 2. .InFie. 1 atterminal A, the transmitter l together with a substationY set2, (consisting of the ordinary substation apparatus, repeating coils,etc.) represents a. transmitting source of speech frequency signals. Theoutput of the set 2 after being `transmitted through the subscribersloop is connected with the distortionv network 3 'having thecharacteristic shown in Fig. 1b, the nature of which will be fullyexplained hereinafter. rlhe output side of the network is connected withtheY amplifier li, which in turn is connected with the line circuit 5,havingtherein a plurality of ampliiiers such as 6 and 7, the purpose ofwhich is to maintainthe level of the trausmi ted signal well above thenoise level of the line, thereby rendering the signal free frominterference due to the noise. lThe' line circuit 5 is connected at thereceiv- .ingterminal B with the amplier `8, kwhich in turn is connectedwitha `distortion network E), the characteristic ofV which,-as shown inFig. 1d, is complementary to that of network 3. The output of thenetwork 9 is connected with a substation set 10 for the detection of thesignal which is impressed upon the receiver 11. The arrangement,representing a one-way circuit, has beenV used for the purpose ofsimplifying the description of the invention, but it is, of course, tobe understood that two way signaling may be effected by well-knownmeans.

The manner in which the system operates in order to insure the sameover-all equivalent at all frequencies is as' follows: As statedhereinbefore, the volume-frequency characteristic of the subset 2 is asshown in Fig. 1a. This characteristic shows that the maximum volume ofenergy is in the neighborhood of 1,000 cycles. Furthermore, let it beassumed that the noise-producing interference has a substantiallyuniform distribution of energy with respect to frequency. I have foundthat by introducing a loss varying with frequency such as lis shown inFig.

1b, 'the output of the amplifier 4 to the line willbe substantiallyconstant for all frequencies within the desired range, sucli, forexample, as that of speech. This loss may be produced by a network 3having the char-V acteristic shown in Fig. 1b. As the curve shows, thenetwork producesV the maximum 'lossat 1,000 cycles, and for frequencieshigh- `er or lower than that frequency, the loss will vary withfrequency in proportion to the changes in volume', as shown in Fig. 1.relative volume transmitted to the line, as shown in Fig. 10, will besubstantially constant, and the line ampliers will be loaded to themaximumr extent at all times. The frequencies remote from the point ofmaximum subset output will be maintained at a higher level than ispracticable if the circuit is operated in the usual manner with themaximum levels limited by the carrying 'capacity of the repeaters for a.relatively narrow frequency range, such as that in the neighborhood of1,000 cycles. Y At the receiving end of such a circuit, a complementarydistortion must be introduced in order to restore the transmitted signalto its original form. This is effected by a network 9 having aAloss-frequency characteristic, such as is represented by Fig. 1d. Aninspection of this figure shows a minimum loss in the neighborhood of1,000 cycles,

and greater loss for frequencies above and below that frequency. It-will therefore be apparent that the energY impressed by the amplifier Son the network 9 will be transmitted through the latter with the variousfrequencies having losses represented by the characteristic 1d, so thatthe resultant wave impressed upon the subset 10 and detected ltherebywill Yhave the characteristic shown in Fig. 1a', which is thecharacteristic of the transmitted signal.

The arrangement shown would also be apessary aparatus for modulation,demodulation, etc.

rIhe arrangement shown in Fig. 2 illustrates the application of theinvention to a radio transmission system. In this figure, thetransmitter 1 at terminal A forms part Vof a substation set 2, theoutput of which is connected with a distortion network 3, which in turnis connected with a radio frequency transmitter 12. In this case, thetransmitter isassumed to be of the high-quality type, 'producingsubstantially no distortion in the air waves comprising its input. Theoutput of this transmitter is connected with the antenna 13, whichradiates the high frequency signal to a receivingantenna 14 assocatedwith the terminal B. At this station, the antenna. 14 is connectedwith aradio receiver 15, the output of which is connected with the subset 10,the connection including a distortion network 9. The output of thesubset is connected with the receiver 11. In the system illustrated, therelative energy distribution at various frequencies of speech may be asrepresented in Qa., wherein it will be seen .that the maximum energy isin the lower frequencies. Byv the use of a network 3 having theloss-frequency characteristic shown in Fig. QZ), the relative volume ofthe transmitter output will besubstantially constant forall thetransmittedfrequencies, as shown in Fig. Qc, At the receiving terminal,the received energy is detected and impressed upon the network j9 whichhas a loss. frequency characteristic as shown in Fig. 2d. As this ligureshows, the greatest loss is throughout the higher frequency range. Theresult, therefore, is that the energy impressed upon the network will bemodified thereby, and the resultant wave impressed by the subset 10 uiorio

of course, that it is capable of embodiment in other and different formsWithout departing from the spirit and scope of the appended claims.

What is claimed is:

1. The method of transmission which consists in distorting the signalWave in order to obtain approximately the same level for 'allfrequencies Within said Wave and introducing a compensating distortionat the receiving end.

2. In a transmission system, the combination With a translating devicehaving a definite load limit, of means `for distorting the signal Waveto be impressed upon said device in order to bring the level of eachfrequency of said Wave up to the maximum level for the apparatusincluded in the system.

3. In a signal transmission system the combination with atransmittingstation of a receiving station, a medium connecting the saidstations, the said transmitting station comprising means to create asignal, and means to distort the said signal to ensure a practicallyuniform amplitude for all frequencies present in the said signal, andthe said receiving station comprising means to introduce a complementarydistortion in the received signal impulse to restore. the impulse to thesame form possessed by it prior totransmission.

4. In a signal transmission system the combination With a source ofenergy the volume of which varies with frequency, of a tiansmissionmedium over which the energy is to be transmitted, an amplifierconnected between the said source and the said medium, a distortingnetwork connected between the said source and the said amplifier tointroduce a loss in the transmitted energy, which loss varies Withfrequency dir-ectly with the variations in volume as created by the saidsource, another distorting network also connected with the said mediumto introduce a loss in the energy as received, which loss ,fvaries Withfrequency inversely with the variations o-f the loss produced by thesaid first mentioned distorting network, thereby restoring to the saidenergy the Wave form possessed by it at the saidsou'rce.

5. The method for maintaining the overall transmission equivalent of'asignal circuit substantially the same for all frequencies Within therange transmitted, which consists in distorting the signal prior totransmission so that the volume of energy for each frequency will be thesame, transmitting the signal as thus distorted, and distorting at thereceiving end of the system by introducing such losses as are necessaryto restore the signal energy to the form possessed by it prior to itsdistortion before transmission.

6. The method for maintaining the overal] transmission equivalent of asignal circuit substantially the same for all frequencies Within therange transmitted, Which consists yquency in 'distorting the? si gnatpriorlto .transmission sefthatithe volume v ofV energy for `eachyfreivillf ybe the same, V introducing a loss into theY circ-uitatitstra-nsmittiiigeiid, which loss varies withy frequencyL directlyWith the variati'oiioffthe' voluine of energy :with v.frei quency,amplifying and transmitting tliesig- `n-al thus modified, receivingthesignal and introducing afloss into tliesaid Vcircuit atthe receivingrend, )vliich loss varies yWith frey quency inyerselyivitli' thecorresponding loss variations at the transmitting end of the saidcircuit.

7. The method for maintainingthe over-all transmission equivalent of asignal circuit substantially the saine for all frequencies Within therange transmitted, Which consists in creating a signal comprising a bandof frequencies, the volume of energy of which Y restored to the formpossessed by it at its creation, and detecting the signal as thusrestored. y

8. in an electrical transmission system, the

combination with a translating device liavin@` a fixed load limit, of asource of oscillatons connected with said device, capable ofsimultaneously producing a plurality of fre-A quencies, and meansinterposed between said device ann said source to substantially equalizethe energy level of all frequencies iinpressed by the source upon saiddevice.

9. In an electrical transmission system, the combination with atransmission circuit for transmitting electrical Waves having therein atranslating device through which electrical energy may be passed at afixed level Without distortion of the electrical Waves, of a sourceofoscillations of a plurality of frequencies andmeans connecting saidsource to the said transmission circuit to substantially equalize theenergy level of the oscillations applied by the source to thetransmission circuit.

l0. rlfhe method of operating an electrical transmission system havingtherein a device havin@- a fixed load limit, Which consists ingeneracing oscillations of a plurality of frequencies, equalizing theenergy level of said oscillations and simultaneously impressing upon atransmission circuit the oscillations of all frequencies as thusequalized for transmission thereover.

l1. The method of operating an electrical transmission svstem havingtherein a translating device having a fixed limit for the ieo transferof energy Without distortion, which consists in creating a signalingWave containing a plurality of frequencies, equalizing the energy levelof all frequencies of said'wave, and impressing the frequencies as thusequalized upon the translating device for transmission over said system.

In testimony whereof, I have signed my name to this specification this23rd day of May, 1927.

HERMAN A. AFF EL.

